A transceiver is an electrical device for sending and receiving electrical signals, in particular signals representative of data in a digital format. A transceiver is disposed at the interface between one system or device and a transmission line coupled to another system or device. For example, a transceiver is used at the output of a computer to transmit the computer output over a wire to a peripheral device, such as a video display terminal or a printer. Many systems that have a central processing unit or microprocessor or microcomputer use transceivers. Such systems may be computers, telephone systems, disk drives, automotive control systems and other industrial system. The transmission line connectible to the transceiver may be a ribbon, coaxial cable or a twisted pair of wires. The latter transmission line is typically coupled to a transceiver that conforms to the RS 485 interface standard.
A transceiver comprises two parts, a driver and a receiver. The driver has an input coupled to the output of a system, typically at the serial output of a UART (universal asynchronous receiver transmitter) device. The UART device converts the normal parallel data bits on a computer bus to a stream of serial bits. That serial stream of bits forms the input signal to the driver circuit of a transceiver. The driver circuit converts the logic level signal of the input to a level compatible with the transmission line. For example, the driver may convert a TTL, ECL or CMOS logic level signal to an RS 485 signal having a differential voltage equal to or greater than 1.5 volts and capable of driving a 54 ohm load.
Prior art driver circuits have been integrated onto a single semiconductor substrate using only bipolar transistors. Such devices have high speed and high drive capability but also consume large amounts of power even when no signal is being driven. Bipolar transistor circuits usually have bias current sources that power the bipolar transistors even when no signal is being driven. In addition, the number of transceivers that could be integrated onto a single semiconductor substrate has been limited by the power consumption of the bipolar transistors. At present, no more than four RS485 type transceivers can be integrated onto a single substrate, but it is desirable to increase the density of integration by adding more, high drive, high speed bipolar drive circuits to the transceiver substrate.
One solution to the power consumption problem has been to use only CMOS transistors in the driver circuit. However, pure CMOS circuits have several limitations. One is component breakdown voltage. CMOS devices simply cannot handle the voltage ranges of input signals to achieve an acceptable range of common mode operation. Common mode operation is a measure of driver performance.
Accordingly, there exists a long felt need for a transceiver with a driver having low power consumption in its quiescent state, a high common mode operation, and high speed.